Optical recording medium and method of manufacturing thereof

ABSTRACT

An optical disk and method of preparing it in which a macromolecular substrate has information bits mechanically formed on its surface and a reflective metallic reflective film is deposited thereover. The grain size of the metallic reflective film is kept to less than 50 nanometers by depositing at a rate of no more than 1.5 nanometers per second.

This is a divisional of U.S. application Ser. No. 06/947,304 filed Dec.29, 1986 now U.S. Pat. No. 4,950,520.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical recording medium, such as anoptical video disk, a compact disk, or the like, in which a reflectivemetallic thin film is formed through evaporation on a surface of asubstrate on which information bits of sub-micrometer size are to beformed. One surface of the substrate is irradiated with light so as toread the information bits by means of reflected light to therebytransmit information, such as an image, sound, characters, figures, andthe like.

2. Background of the Invention

Conventionally, an optical video disk, an example of the opticalrecording medium of the kind described above, is formed as shown inFIGS. 1 and 2. The method of manufacturing the recording medium will bedescribed hereunder.

In the drawings, a substrate 1 is made of a macromolecular compoundconsisting of components having a polar group at their side chains, forexample, acrylic resin or epoxy resin. Information bits 5 are formed onthe surface of the substrate 1 by means of a stamper or the like.

A thin film 2 made of aluminum or an alloy of the same is formed throughevaporation as a reflective film on the surface of the substrate 1having the information bits 5 formed thereon so as to reflect lightrays, such as laser light or the like, which enter the substrate 1 fromits opposite surface.

The thus evaporation-formed thin film 2 is coated with a protective film3. Then an adhesive 4 is applied on the protective film 3. Two of thethus prepared substrates 1 are stacked together with the respectivesurfaces each applied with the adhesive 4 on their joining surfacesbeing stuck together. Thus, an optical video disk is fabricated.

In one method, the evaporation of aluminum is performed in a highvacuum, for example, at 10⁻⁵ torr and the aluminum thin film 2 is formedat a high speed, for example, at an evaporation speed of not lower than2 nanometers per second. This method is recommended for general use inorder to provide a beautiful appearance. In fact, this method is used incommercial production.

In order to read the information bits 5 by irradiating the surface ofthe substrate 1 with light rays such as laser light or the like, thelight rays such as laser light or the like are concentrated into a spotof about 1 micrometer in diameter to irradiate the bit surface with thelight spot to thereby read the information by analyzing the reflectedlight from the bit surface.

If the evaporation of aluminum is performed in a high vacuum and at ahigh speed as described above, generally, the crystal grain size of thethin film is generally equal to or larger than 50 nanometers.

If the grain size is larger than about 50 nanometers, extremely smallpeelings are generated between the substrate 1 and the aluminum thinfilm 2 owing to time aging to thereby generate blisters 6 ofsubstantially the same size as the information bits 5 as shown in FIG.3.

Such blisters 6 are important in the case where information is read froman optical recording medium as described above. That is, the disk issubject to a defect that faults such as erroneous reading of propersignals occur.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to eliminate thedisadvantages discussed above in the conventional optical recordingmedium.

It is another object of the present invention to provide an opticalrecording medium in which the generation of the foregoing blistersarising from aging or heating is prevented from occurring and thereoccurs no trouble of color flash generation and/or erroneous readingfaults.

It is a further object of the present invention to provide a method ofmanufacturing such an optical recording medium as described just above.

In order to achieve the above-mentioned objects, according to a firstaspect of the present invention, the optical recording medium comprisesa substrate made of a macromolecular compound consisting of componentshaving a polar group in their side chains such as acrylic resin.Information signal bits are formed on a surface of the substrate and athin film is formed on the surface of the substrate over the informationsignal bits. The thin film is made of a material selected from a metalgroup consisting of aluminum, an alloy of aluminum, and the like. Acrystal grain size of the thin film is selected to be not larger than 50nanometers.

According to a second aspect of the invention, the method ofmanufacturing an optical recording medium comprising the steps ofpreparing a substrate carrying information signal bits formed on itssurface thereof and forming a reflective thin film by evaporation ofaluminum, an aluminum alloy, or the like, onto the surface of thesubstrate over the information signal bits at a speed not higher than1.5 nanometers per second.

Hereinafter, the term "color flash" is defined as the phenomenon inwhich a picture reproduced from an optical recording medium flickers asif stars were twinkling in the reproduced picture. The color flasharises from faults in a recording surface of the optical recordingmedium.

The foregoing and other objects, features and advantage of the inventionwill be apparent from the following description taken in connection withthe accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of an optical video disk;

FIG. 2 is an enlarged section of a part of the same, and

FIG. 3 is an enlarged section showing a blister of a thin film.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention has originated from the discovery of the fact thatthe color flash or erroneous reading is caused during reading on anoptical recording medium by the previously described blisters 6 of thealuminum thin film 2 of FIG. 3.

It has been found that the blisters 6 have a close relationship with thecrystal grain size of the thin film 2 formed on the substrate 1.

That is, there were prepared two kinds of substrates respectivelyprovided with aluminum thin films 2 each having a film thickness of40-80 nanometers but respectively having a grain size not larger than20-30 nanometers and a grain size not smaller than 50-80 nanometers.

Those substrates were left in a room for a long time and some of themwere heated and dampened to accelerate deterioration. The result wasthat no blisters were generated in the thin film of small crystal grainsize while blisters were generated in the thin film of large crystalgrain size.

The reason why such a phenomenon occurs is considered to be as follows.There is a difference in the coefficient of thermal expansion betweenthe aluminum thin film 2 and the substrate 1 due to the difference inthe quality of material between the thin film 2 and the substrate 1.This difference in the coefficients causes a difference inexpansion/contraction between the thin film 2 and the substrate 1 whichappears as a distortion. This distortion is difficult to disperse if thecrystal grain size is large, so that the thus generated stress overcomesthe bonding force between the substrate 1 and the thin film 2 to therebypartially peel the thin film 2 from the substrate 1 to generate theblisters 6.

In the thin film of the small crystal grain size, on the other hand, itis considered that the foregoing distortion is easily dispersed byslippage of crystal or the like, so that there occurs no stresssufficient to peel the thin film 2 from the substrate 1.

It has been found that the aluminum thin film 2 having such a smallcrystal grain size can be formed when the evaporation speed is selectedto be equal to or lower than (1.5 nanometers) per second and the degreeof vacuum is set to the order of 10⁻⁴ torr.

In view of the foregoing point, according to the present invention, thecrystal grain size of the reflective thin film 2 of a group of metalformed on the surface of the substrate 1 having the information bits 5formed thereon is selected to be not larger than 50 nanometers.

In an optical video disk in which the thin film 2 is formed in such acondition as described above, no blister 6 is generated so that apicture of good quality can be obtained even under a difficult conditionsuch as heating and/or dampness.

Accordingly, reliability during aging is improved.

Particularly, if the substrate 1 is made of a macromolecular compoundconsisting of components having a polar group in their side chains suchas acrylic resin or epoxy resin, the polar group has a strong bondingforce with aluminum so that the blisters 6 are apt to be generated. Inthis regard, however, the reduction in crystal grain size is effectiveto prevent the blisters 6 from occurring.

Although the invention has been described for an aluminum thin film, themethod can be similarly applied to the formation of thin films of ametal group having reflective property such as an aluminum alloy,silver, gold, or the like, to be used for the same purpose.

The present invention can be realized in manufacturing not only theoptical video disks as described above but other optical recording mediasuch as compact disks, DRAW disks, E-DRAW disks, and so on.

Further, the method according to the present invention is extremelyeffective for the prevention of the foregoing deterioration when used inmanufacturing an optical recording media having substrates made of amacromolecular compound of components having a polar group at their sidechains. Such macromolecular compounds are acrylic resins or epoxyresins, in which the polar group has a strong bonding force withaluminum.

As described above, according to the present invention, no blister isgenerated in the reflective thin film of a metal group during aging ortemperature excursions, and so on, so that there occurs no deteriorationin signal such as color flash, erroneous reading of information, or thelike. Reliability is thus improved.

In the case of using the crystal grain size as described above accordingto the invention, the difference in expansion/contraction between thesubstrate and the thin film owing to time aging, heat impact, or thelike, is dispersed to thereby prevent blister and peelings of the thinfilm from occurring. Therefore, any color flash and any erroneousreading of information signals due to blisters or peelings can beprevented from occurring to thereby extremely improve the reliability.

The manufacturing method can be carried out only by lowering the speedof evaporation for forming the thin film below the conventional speed aswell as can be carried out in a low vacuum, so that the productivity canbe maintained.

Although the generation of blisters has been prevented, conventionally,by means of fine control on the external conditions such as water, orthe like, the control becomes unnecessary in the manufacturing methodaccording to the invention in which the crystal grain size is madesmall, so that the productive efficiency can be significantly improved.

What is claimed is:
 1. A method of manufacturing an optical recordingmedium comprising the steps of:preparing a substrate carryinginformation signal bits formed as indentations on a surface thereof; andevaporating a metallic reflective film on said surface of said substrateover said information signal bits at a speed not higher than 1.5nanometers per second to prevent the formation of readout disruptiveblisters by said reflective film.
 2. A method as recited in claim 1,wherein said reflective film is a material essentially consisting ofaluminum or an aluminum alloy.
 3. A method as recited in claim 1,wherein said evaporating step is carried out in a vacuum ofapproximately 10⁻⁴ torr.
 4. A method as recited in claim 1, furthercomprising a step of applying a protective film on said surface of saidsubstrate.